Ultrasonic nonlinear imaging at fundamental frequencies

ABSTRACT

Nonlinear tissue or contrast agent effects are detected by combining echoes from multiple, differently modulated transmit pulses below the second harmonic band. The received echoes may even overlap the fundamental transmit frequency band. The modulation may be amplitude modulation or phase or polarity modulation, and is preferably both amplitude and phase or modulation. The present invention affords the ability to utilize a majority of the transducer passband for both transmission and reception, and to transmit pulses which are less destructive to microbubble contrast agents.

This invention relates to ultrasonic diagnostic imaging systems and, inparticular, to ultrasonic diagnostic imaging systems which imagenonlinear signals in the fundamental frequency band.

U.S. Pat. No. 5,879,303, of which I am a co-inventor, describes methodsand apparatus for doing harmonic ultrasound imaging. As explained in mypatent, an ultrasonic wave can be transmitted at a fundamental frequencyto give rise to harmonic echo signals, in particular at the secondharmonic, from two distinct sources. One is the nonlinear behavior ofmicrobubble contrast agents. When these microbubble agents areinsonified by the transmit wave, they will oscillate or resonatenonlinearly, returning a spectrum of echo signals including those at thesecond harmonic of the transmit frequency. The strong harmonic echocomponents uniquely distinguish echoes returning from the microbubbles,which can be used to form B mode or Doppler images of the bloodflowinfused by the contrast agent. The other source of harmonic echo signalsis the nonlinear distortion which ultrasonic waves undergo as theytravel through tissue. The echoes returned from these distorted wavesmanifest harmonic components developed by this distortion.

My aforementioned U.S. patent describes two ways in which the harmoniccomponents of these echo signals may be detected. One is by use of ahighpass filter, which will pass signals in the harmonic band whileattenuating the stronger echo components in the fundamental band. Theother way is by transmitting two or more pulses of opposite phase orpolarity and combining the echoes received in response from the twopulses. The fundamental components, being of opposite phase or polarityby reason of that characteristic of the transmit pulses, will cancel.The harmonic components of the combined echoes, being quadratic innature, will additively combine, leaving the separated second harmonicsignals.

As discussed in my aforementioned patent, harmonic signals areadvantageous in many imaging situations because of the distinctive wayin which they identify echoes returned from harmonic contrast agents.When used without contrast agents the tissue harmonic signals areadvantageous because their development within the body eliminates muchof the clutter caused by nearfield effects. However, harmonic signalsare of a significantly lower amplitude than the fundamental signalechoes, providing lower signal to noise ratios and requiring greateramplification. In addition, harmonic signals require the use ofrelatively low frequency transmit pulses so that the second harmonicecho signal will be of a frequency which can be received within thetransducer's passband. Generally, the transmit signal will be centeredat the lower end of the transducer's passband so that the secondharmonic return signal will be below the upper cutoff of the transducerpassband. This can place the transmit and receive signals at theextremes where broadband signals will experience attenuating rolloff. Italso mandates lower frequency transmit signals, which can be moredisruptive to microbubble contrast agents than higher transmitfrequencies would be. Accordingly it is desirable to be able to overcomethese deficiencies and limitations of harmonic imaging.

In accordance with the principles of the present invention, thenonlinear signals returned from tissue and contrast agents are detectedin the fundamental frequency band rather than at harmonic frequencies.In a preferred embodiment the nonlinear signals are detected by anamplitude modulated two (or more) pulse technique. Preferably thetransmit pulse waveforms are of opposite phase and polarity and ofdifferent amplitudes. Upon reception the echoes are normalized for thedifferent transmit amplitudes and combined and the signals within thefundamental band are used for imaging.

In the drawings:

FIG. 1 illustrates conventional transmit and harmonic receive spectrawithin a transducer passband;

FIG. 2 illustrates the location of a harmonic spectrum at the upperlimit of the transducer passband;

FIG. 3 illustrates the fundamental transmit and second harmonic receivespectra of an embodiment of the present invention;

FIG. 4 illustrates the spectrum of fundamental nonlinear echo signalsresulting from the transmit spectrum of FIG. 3;

FIG. 5 illustrates receive passbands for the nonlinear echo signals ofFIG. 4;

FIGS. 6a-6 g depict waveforms illustrating the principle of pulseinversion harmonic separation;

FIGS. 7a-7 g depict waveforms illustrating the principle of nonlinearecho signal detection in the fundamental band in accordance with theprinciples of the present invention;

FIG. 8 illustrates the transmit and receive passbands of one embodimentof the present invention;

FIG. 9 illustrates the transmit and receive passbands of anotherembodiment of the present invention; and

FIG. 10 illustrates an ultrasonic imaging system constructed inaccordance with the principles of the present invention.

Referring first to FIG. 1, typical fundamental and harmonic spectra ofan ultrasound system are shown. This drawing illustrates a passband 10of an ultrasonic transducer/beamformer which transmits the fundamentalfrequency pulses or waves, and receives the harmonic echo signals. Inthis example the transducer has a passband extending from 1 to 3 MHz.When the same transducer is to be used for both transmission andreception, both the fundamental transmit pulse and the harmonic receiveechoes must be encompassed within the passband 10 of the transducer. Inthis example the transmit pulse exhibits a passband 12 which is centeredaround 1.25 MHz. Second harmonic echo signals will be received in apassband 14 centered around 2.5 MHz. It is seen that because thetransmit band 12 is at the lower end of the transducer passband 10, theharmonic receive passband will fall in the upper portion of the passband10 and thus both transmission and reception can be performed by thisparticular transducer.

As FIG. 1 shows, in order to get both the fundamental band 12 and theharmonic band 14 within the same transducer passband it is oftennecessary to fit one or the other or both of the transmit or receivepassbands at one of the cutoff extremes of the transducer passband. FIG.2 shows another example of this, in which the fundamental transmit band16 is centered about a frequency of 1.5 MHz and occupies the entirelower half of the transducer passband 10. The transmit band 16 thus isfor a more broadband transmit signal than that which is transmitted bythe passband 12 in FIG. 1, providing improved image detail and quality.However the second harmonic receive passband 18 for this transmit pulseis centered at 3 MHz at the upper extreme of the transducer passband 10.In this example the center of the harmonic band is at the upper cutoffof the transducer band, resulting in significant attenuation of signalsin the upper portion of the band 18. Thus, broadband harmonic imaging islimited by this transducer passband.

The passbands used in a first embodiment of the present invention isshown in FIGS. 3-5 using the same transducer passband 10 as in theprevious drawings. Two or more differently modulated broadband transmitpulses having a passband 20 are transmitted along each scanline in theimage field as shown in FIG. 3. As this drawing shows, the fundamentaltransmit pulses centered at 2 MHz will elicit second harmonic echosignals in a band 22 centered at 4 MHz. These harmonic echo frequenciesare outside the transducer passband 10 and hence will be beyond thefrequency range of the transducer. However the transmit pulses will alsoelicit echoes in the fundamental frequency band 20 and beyond, as shownby the dashed line receive passband 24 in FIG. 4. As will be discussedbelow, these fundamental frequencies can exhibit varying degrees oflinear and nonlinear characteristics depending upon the presence ofnonlinear reflectors such as contrast agents in the image field. Thenonlinear characteristics are extracted and the linear fundamentalcomponents canceled by combining the echo signals in a chosen receivepassband such as receive bands 30, 32 or 34 as shown in FIG. 5. Thereceive band can start at the higher frequency location 32 at theinitial reception of shallow depth echoes, then be moved dynamically tothe lower frequency position 34 during reception to account for theeffects of depth dependent frequency attenuation. By virtue of themodulation of the transmit pulses and the nonlinearity of the reflectorsthe correlated linear characteristics in the fundamental band willcancel and the uncorrelated nonlinear characteristics in the fundamentalband will not, leaving an echo signal component which is a measure ofthe nonlinearity of the fundamental frequency echoes. Thus, nonlinearcomponents are detected which are below the second harmonic frequency.

The use of these nonlinear components below the second harmonicfrequency provide several advantages. For one, most or even all of thetransducer passband can be used for transmission. This enables the useof broadband transmit pulses which will result in broadband echo signalsfor finer and more subtle image detail. There is no need to constrainthe transmit pulses to a narrow range of the transducer passband.Another advantage is that the transmit and receive passbands can both bemore centered in the transducer passband, away from the rolloff at theextremes of the transducer passband. Higher transmit pulse frequenciesand shorter duration pulse bursts may also be used, providing advantagesin the form of reduced microbubble destruction.

FIG. 6 illustrates the principles of pulse inversion transmission andreception. A first transmit pulse 40 (FIG. 6a) has a first phase orpolarity characteristic and a second transmit pulse 50 has an secondphase or polarity characteristic (FIG. 6b). In this example both pulsesare shown as a single cycle of a waveform and the second pulse 50 is theinverse of the first. The first transmit pulse 40 returns echo signalsfrom a nonlinear system such as a microbubble which have a fundamentalfrequency component 42 (FIG. 6c) which follows the phase or polaritymodulation of the transmit pulse, and a second harmonic component 44(FIG. 6e). The second transmit pulse 50 returns echo signals from thenonlinear system which have a fundamental frequency component 52 (FIG.6d) which also follows the phase or polarity of the transmit pulse, anda second harmonic component 54 (FIG. 6f). When the echo signals from thetwo transmit pulses are combined the fundamental frequency componentswill cancel each other and the harmonic components will additivelyreinforce each other by reason of the quadratic nature of harmonics,leaving a detectable second harmonic component 60 (FIG. 6g). Thus, thesecond harmonic components have been separated from the fundamentalfrequency components of the echo signals.

FIG. 7 shows waveforms illustrating the principles of the presentinvention. Like pulse inversion the technique of the present inventionuses multiple, differently modulated transmit pulses to separatenonlinear signal components. FIGS. 7a and 7 b show two exemplarytransmit pulses 70 and 80 which are of different amplitudes. In thisexample the first transmit pulse 70 is twice the amplitude of the secondtransmit pulse 80, although other amplitude relationships may beemployed. In this example the two transmit pulses are also of oppositephase or polarity. Transmit pulse 70 elicits different fundamentalfrequency echo signal characteristics from nonlinear and linear targets.For instance, if the echo is returned from a nonlinear contrast agent,the nonlinear behavior of the microbubbles when insonified will return afundamental frequency echo waveform 72 as shown in FIG. 7e which isnonlinearly related to the transmit waveform. If the echo is returnedfrom a linear reflector such as tissue, a fundamental frequency echo 74as shown in FIG. 7c results, which is seen to be linearly related to thetransmit pulse.

The second transmit pulse will elicit fundamental frequency echo returnsfrom nonlinear and linear reflectors as shown in FIGS. 7f and 7 d. Anonlinear system such as a microbubble will return an echo 82, which isnonlinearly related to the transmit waveform 80. Since the transmitpulse 80 is of a lesser amplitude than the first pulse, the microbubblewill behave differently by reason of the different level ofinsonification. A linear reflector returns an echo 84 which is seen tobe linearly related to the lesser amplitude transmit pulse 80.

The first and second echo signals are normalized to account for thedifferent transmit pulse amplitudes. When the two transmit pulses differby a factor of two as they do in this example, the echoes from thesecond pulse would be amplified by a factor of two, for instance. Whenthe corresponding echoes are combined after normalization, it can beseen that the linear echoes will cancel as shown by line 94 in FIG. 7g.The echoes returned from the nonlinear reflectors will partially cancelbut leave a difference which is a manifestation of the differentnonlinearities of the echoes as shown by waveform 92 in FIG. 7h. That isbecause the nonlinear characteristics of echoes 72 and 82 are notequalized by the linear normalizing and will leave a residual signalafter combining because of the decorrelation of the two nonlinear echosignals. The nonlinear effects in the echoes are not linearly related tothe difference in pulse amplitude of the two transmit pulses. This meansthat the normalization, which will equalize the two linear echoes 74 and84 and result in cancellation, will not equalize the echoes from thenonlinear reflectors. The oscillation of microbubbles when insonified bypulses of different amplitudes is nonlinear and more complex than justthe amplitude difference. Furthermore, the microbubbles can be disruptedby the first pulse so that the microbubbles interrogated by the secondpulse have a different character than those encountered by the firstpulse. The combination of these different nonlinear and behavioralcharacteristics of a nonlinear system provide the ability to clearlydistinguish echoes from nonlinear systems in the fundamental frequencyband.

FIG. 8 illustrates the passbands of another embodiment of the presentinvention. The fundamental frequency transmit pulse band is shown by thelined passband 200 centered at 2 MHz. The receive band 202 is centeredat 2.7 MHz and greatly overlaps the transmit passband to receive echoesin a portion of the received echo band 204. In the prior art great painswere taken to transmit pulses in a band which did not overlap thereceive band so that the second harmonic signals could be cleanlyseparated from the fundamental frequency signals. In the presentinvention, where it is nonlinear components at and around thefundamental band which are of interest, this is not a problem.

FIG. 9 illustrates yet another embodiment of the present invention wherethe receive band 212 is below the transmit pulse band 210. In thisexample the transmit band is a high frequency band centered about 3.3MHz. The high frequency transmit signals result in better resolution inthe echo signals. Locating the transmit band at the upper end of thetransducer passband 10 obviously cannot be done when trying to containboth the fundamental and second harmonic bands in the transducerpassband. Thus, the present invention will afford better imageresolution than prior art harmonic systems. The receive band 212encompasses some of the echo signal frequencies in the echo signal band214 and is centered at 2.7 MHz in this embodiment. show, the duration ofthe 3.3 MHz pulse is considerably less than that of the lower frequencyburst, causing less microbubble disruption. The 3.3 MHz pulse, being ofa higher frequency, will also cause less disruption than the lowerfrequency pulse for this additional reason. Furthermore, the loweramplitude transmit pulse (FIG. 7b) will cause less disruption than thehigher amplitude transmit pulse. While the different amplitude pulsescan be transmitted in either order, transmitting the lower amplitudepulse as the first pulse will cause less disruption to the microbubblefield that is encountered by the second, higher amplitude pulse. The twotransmit pulses can also differ in amplitude only or in phase orpolarity only, but the combination of the two modulation differences,both amplitude and phase or polarity, provides better nonlineardecorrelation and thus better nonlinear sensitivity.

FIG. 10 illustrates an ultrasound system in block diagram form which isconstructed in accordance with the principles of the present invention.An ultrasound scanhead 100 including an array transducer 102 isconnected to a transmit/receive (T/R) switch 104. A central controller110 responsive to a user interface (not shown) sets the frequency,amplitude, and phase or polarity of the transmit pulse. A transmitter106 transmits the pulses set by the central controller by way of the T/Rswitch, exciting elements of the transducer array in a timed sequence totransmit appropriately steered and focused beams. The echoes received bythe transducer array 102 are coupled by the T/R switch to a receivebeamformer 108. In this example the beamformer is shown as a multilinebeamformer which, under control of the central controller, produces twospatially adjacent receive lines of coherent echo signals A and B. Theecho sequences produced in response to the first transmit pulse arestored in line buffers 120 and 130 for the A and B multilines,respectively. The echo sequences produced in response to the secondtransmit pulse, in this example a lower amplitude pulse, are multipliedby a factor of two in multiplier circuits 122 and 132 when thedifference in amplitudes is a factor of two. The multipliers can beeasily implemented for multiplication by two in a digital system byshifting the echo signal values one bit to the left to multiply by two(or one bit to the right to halve a signal which is twice the othersignal amplitude). After this normalization the echo sequences arecombined in summers 124 and 134 respectively to separate the nonlinearfundamental components. When these summers are set for subtraction thelinear components will be emphasized for oppositely phased or poledtransmit pulses. The echo signals are then filtered by filters 126 and136. In a preferred embodiment these filters are quadrature bandpassfilters as described in my aforementioned patent, to produce quadraturesignal components and also bandpass filtering for the receive passband.

The echo signals may be processed by a B mode processor 140, a Dopplerprocessor 150, and/or a contrast signal processor 160. The B modeprocessor will amplitude detect the echo signals in the production ofimage signals, and the Doppler processor will process ensembles of echosignals to produce image signals of tissue or flow motion. The contrastsignal processor is similar to the previous processors, generally with athreshold which separates contrast harmonic signals from tissue harmonicsignals. Contrast agents can be displayed in either Doppler or B modeformat. Image signals from the three processors are coupled over animage signal bus 172 to a scan converter 170, which interpolates theimage signals and puts the scanlines in the desired image format. Theimage information can be applied to a video processor 180 for display ofa two dimensional image on a display 190. The image information can alsobe formed into three dimensional presentations by 3D image rendering182. Three dimensional images are stored in a 3D image memory 184 anddisplayed on the display 190 by way of the video processor 180.

Other variations will be apparent. While the embodiment of FIG. 10provides the obvious benefit of 2×multiline, which is a doubling of thescanline density or a halving of the framerate, these factors can befurther improved by interpolation. For example, after transmitting twopulses of different modulation characteristics to produce two A linesequences of different characteristics and two B line sequences ofdifferent characteristics, an A line sequence of one characteristic canbe combined with a B line sequence of the other characteristic tointerpolate a further line of nonlinear or linear signals between the Aand B lines. This could be done by combining the outputs of multipliercircuit 122 and line buffer 130 in an additional summer, for instance.By using more than two transmit pulses for a scanline motional effectscan be reduced as explained in U.S. patent application Ser. No.09/434,328 entitled “Ultrasonic Pulse Inversion with Reduced MotionalEffects”, of which I am a co-inventor. The system of FIG. 10 can be usedfor continuous realtime contrast imaging as described in U.S. patent[application Ser. No. 09/302,063] entitled “Realtime Ultrasonic Imagingof Perfusion Using Ultrasonic Contrast Agents”, of which I am aco-inventor. Embodiments of my invention can be used for detecting thenonlinear effects of numerous nonlinear reflectors, such a microbubblecontrast agents and nonlinear effects due to pulse travel throughtissue. The band selection performed by filters 126 and 136 can beomitted or performed by FIR filters or in the wall filter of the Dopplerprocessor 150.

What is claimed is:
 1. A method for nonlinear ultrasonic imagingcomprising: transmitting first and second fundamental frequency pulsesto a nonlinear target which are differently modulated in at least one ofamplitude, phase or polarity; receiving echoes from said nonlineartarget at a frequency below the second harmonic frequency of saidfundamental frequency in response to said transmitted pulses; andcombining said received echoes which are below the second harmonicfrequency to produce signals embodying a nonlinear effect of saidnonlinear target; and using said nonlinear effect signals to produce anultrasound image.
 2. The method of claim 1, wherein said transmit pulsesare differently amplitude modulated.
 3. The method of claim 1, whereinsaid transmit pulses are differently modulated in phase or polarity. 4.The method of claim 1, wherein said transmit pulses are differentlymodulated in amplitude and in phase or polarity.
 5. The method of claim2, further comprising normalizing said received echoes to account forsaid amplitude modulation difference.
 6. The method of claim 1, whereinsaid transmitted pulses occupy a fundamental frequency band, and whereinsaid receiving comprises receiving echoes from said nonlinear target ata frequency which is within said fundamental frequency band in responseto said transmitted pulses.
 7. The method of claim 1, wherein saidtransmitted pulses occupy a fundamental frequency band, and wherein saidreceiving comprises receiving echoes from said nonlinear target at afrequency which is below said fundamental frequency band in response tosaid transmitted pulses.